1. Field of the Invention
This invention relates broadly to audio speaker systems. More particularly, this invention relates to a horn-type audio speaker system which limits the dispersion of sound output from the speaker system.
2. State of the Art
A number of speaker systems are known for focusing sound. Sound focusing speaker systems have particular application where it is desired to prevent sound emitted by one speaker system from interfering with sound emitted by another speaker system. These speaker systems are also useful for xe2x80x9clistening stationsxe2x80x9d where it is desired that only listeners at the listening station be able to hear the sound from the speaker system.
Typically, a sound focusing speaker system uses a concave lens and a speaker directed into the concave lens. Ideally, the lens reflects sound from the speaker such that the sound reflected is confined to a desired area. For example, U.S. Pat. No. 5,268,539 to Ono discloses a partial ellipsoid sound lens having a speaker at one focus of the lens. Proper placement of the speaker at one focus results in the sound being reflected by the lens and focusing at the second focus of the ellipse, where the listener is ideally positioned. Unless a listener has his or her ears located at the second focus, listening will not be optimal. In addition, because sound is reflected back toward the second focus from many angles, sound will overshoot the second focus, and failing to be contained, will strike floor surfaces and disperse. The dispersion of sound will provide auditory interference to others in the vicinity of the ellipsoid sound lens.
U.S. Pat. No. 5,532,438 to Brown discloses a sound lens speaker system similar to the Ono system. The Brown system includes a spherical dome and left and right channel speakers (each speaker reproducing the same frequency range) directed into the dome. The speakers are oriented such that sound from the speaker reflects off the inside of the dome and is purportedly focused in stereo at the listeners ears. The Brown system suffers from the same drawbacks as the Ono system. The ears of the listener must be particularly positioned at a particular height relative to the dome to accurately hear the reflected sound. In addition, the speakers will cause sound to spill over outside the spherical dome. Furthermore, the spherical shape of the dome will likely further propagate uncontrolled sound scatter outside the dome.
Museum Tools of San Rafael, Calif., offers a sound lens speaker system under the name Secret Sound(copyright) which includes a parabolic sound lens and a speaker located at the focus of the parabolic lens. The speaker radiates sound upward into the sound lens and the sound lens then focuses the sound into a substantially vertical beam of sound, thereby reducing the amount of sound which is uncontrollably scattered. However, contrary to the Secret Sound(copyright) literature, the Secret Sound(copyright) sound lens is not designed to handle a full spectrum of humanly audible sound. The curvature and size of the parabolic lens is not optimized to accurately reflect both high and low frequency sound waves.
An additional complicating factor, which has not adequately been taken into consideration in the prior art, is that sound produced from each of the speakers into its respective lens has a spherical wavefront, which naturally disperses in an uncontrollable manner. None of the sound lens speaker systems of the prior art controllably confines the spherical wavefront of the sound it produces. Moreover, in each of the speaker systems of the prior art, the speakers are incapable of reproducing a broad spectrum of sound frequencies. Also, in all of the above speaker systems sound is radiated by the sound lens because the sound lens is formed from a single layer of material and the exterior surface of the sound lens is not acoustically isolated from the interior surface of the sound lens. As a result, sound waves produced by a speaker causes the sound lens to which it is coupled to radiate spurious sound.
It is therefore an object of the invention to provide a speaker system which is suitable for providing sound confined to a relatively small listening zone.
It is another object of the invention to provide a directional speaker system which limits the area of dispersion of sound output from the speaker system.
It is a further object of the invention to provide a directional speaker system which reproduces a broad frequency spectrum of sound.
It is an additional object of the invention to provide a directional speaker system having a horn-type configuration designed to optimally, controllably direct a broad spectrum of sound frequencies such that sound produced by the speaker system is confined to a relatively small area.
It is also an object of the invention to provided a speaker system which substantially converts the sound wavefront from spherical wavefront to a linear wavefront for improved directionality.
In accord with these objects which will be discussed in detail below, a directional speaker system which manipulates and confines sound waves includes a suspendable elongate enclosure having an inner surface, an outer surface, an open end, and a closed end, a speaker driver directed toward the open end and mounted near the closed end of the enclosure, an in-line phase plug mounted in front of the speaker driver, and a substantially frustoconically shaped wave guide (horn) provided between the speaker driver and the open end of the speaker enclosure. The inline phase plug is preferably cylindrically shaped and has a plurality of channels. The wave guide has an inner surface, an outer surface, a first open end provided near the speaker driver, a second open end located near the open end of the speaker enclosure, and a length preferably at least twice a dimension of the second open end. In addition, the open end of the enclosure is preferably provided with an acoustic trap; i.e., an acoustically absorbent ring of material provided around the inner perimeter of the open end. The absorbent ring preferably has a constricted opening relative to the second opening of the wave guide.
According to preferred aspects of the invention, an acoustically absorbent material is provided between the speaker enclosure and the wave guide. In addition, the speaker driver is preferably spaced apart from the mount to acoustically isolate the speaker driver from the mount and reduce acoustic vibration between the speaker driver and wave guide and the outer enclosure. According to the several embodiments of the invention, the enclosure can be generally cylindrical, rectilinear, or conical with a regular or irregular, e.g., undulating, surface. In addition, the wave guide can be have an arcuate side wall, e.g., the wave guide can be concave or convex.
According to one preferred embodiment of the invention, the speaker enclosure and the wave-guide are integrally molded or otherwise formed such that a hollow is provided between the inner surface of the enclosure and the outer surface of the wave guide. The hollow is then at least partially filled with the acoustically absorbent material. At the open end of the speaker enclosure, the enclosure is provided with a plurality of preferably evenly spaced apart openings. Sound waves propagated by the speaker driver in the forward direction exit the wave guide, while sound waves propagated rearward are out-of-phase with the forwardly directed sound waves and travel through the hollow to exit the openings. At the perimeter of the wave guide, the forward and rearward propagated sound waves are theoretically canceled by each other to confine the remaining sound waves and control dispersion. The preferred speaker system further comprises a wave guide phase plug having at least one cylindrical disc situated concentrically within the wave guide, a highly directional horn tweeter located beneath the wave guide phase plug and directed through the open end of the speaker enclosure, and a baffle plate provided above the acoustic trap. The wave guide phase plug and baffle plate operate to further flatten the wavefront, while the horn tweeter replaces some high frequencies lost due to wavefront manipulation.